The manufacture of high-information-content flat-panel displays for the computer and video markets imposes stringent requirements for the substrate that supports the electronic display elements. A substrate is required that is rigid, nearly atomically smooth and flat, transparent with very low optical distortion, resistant to thermal-stress buildup, with the thermal expansion coefficient close to that of silicon, that has the ability to accept surface chemical modifications and coatings (including strong chemical washes), and that is thermally stable at post-forming processing temperatures of 600.degree. C. to 800.degree. C. for tens of hours.
To meet these requirements, glass manufacturers have developed suitable glasses and manufacturing techniques for such glasses. For example, Corning, Inc. has developed a new class of alkaline earth aluminoborosilicate glass substrates produced via the fusion draw process in which a highly viscous glass melt is poured over a "fusion pipe" that separates the stream into two smooth melt sheets. The sheets are rejoined (fused) below the pipe and cooled, producing a flat glass ribbon with two virgin sides. See, D. M. Moffat, "Flat Panel Display Substrates," Mat. Res. Soc. Symp. Proc., Vol. 345, 1994, pp. 163-174; D. M. Moffat, "Glass Substrates for Flat Panel Displays," MRS Bulletin, Vol. 21, 1996, pp. 31-34. Such fusion glasses can meet the requirements noted above, producing extremely flat, thin (less than 0.5 mm thick) glass sheets. However, due to the high viscosity of the glass melts involved, the fusion draw process is limited by a relatively slow production rate. In addition, only a very limited number of glass compositions can be handled by the fusion draw process because of the extreme viscosity restrictions of the process.
The float process is the dominant method of flat glass manufacturing for architectural applications. As commercially developed, the process allows economic production of relatively wide (e.g., four meters) glass ribbons at rates of up to kilometers per day. The float process has been optimized for manufacture of soda-lime-silica (NCS) glass, which is well suited for architectural applications. However, NCS float glass is not generally compatible chemically with the electronic elements of high resolution flat panel displays, and typically cannot meet the rigorous tolerances at the thicknesses required for such flat panel displays. Conversely, the conventional float process is not suited to the manufacture of glasses such as aluminoborosilicate which must be formed at higher temperatures than NCS glass.
In the typical commercial float process, an NCS glass melt is poured from the melting-conditioning furnace onto a bath of molten tin. See, generally, W. C. Hynd, "Flat-Glass Manufacturing Processes," in Glass Science and Technology, Vol. 2, D. R. Uhlmann and N. J. Kreidel, Eds., Academic Press, New York, 1984, pp. 83-100. The glass is applied to the liquid tin at a temperature of about 1100.degree. C. (a silicate melt viscosity of 10.sup.3 Poise), resulting in a nearly perfectly flat ribbon of molten glass being formed on the liquid tin. Typical commercial molten tin float baths have dimensions on the order of 5 meters wide and 50 meters long. As the glass melt ribbon travels the length of the float bath, with the molten tin acting as a conveyor, it is subjected to a controlled cooling gradient, reducing its viscosity until it is cool and stiff enough (about 600.degree. C., 10.sup.10 p) to be transferred to an annealing lehr. Depending on the thickness of the glass, residence time of the ribbon on the tin bath can vary from 150 seconds for a 2 mm thick sheet to 880 seconds for a 12 mm thick sheet. Pure tin is typically used for the float medium because of its unique combination of properties of low melting point (232.degree. C.), high boiling point (2623.degree. C.), low vapor pressure at the processing temperatures used (about 10.sup.-6 atmospheres at 1100.degree. C.), and a density (7 g-cm.sup.-3) that is greater than that of an NCS glass melt (2.5 g-cm.sup.-3) so that the glass will float. The bath structure typically includes a refractory lined (e.g., ZrO.sub.2 --Al.sub.2 O.sub.3 --SiO.sub.2) steel tank. The bath is fully enclosed above the tank and contains a reducing gas mixture ("forming gas"--95% N.sub.2 -5%H.sub.2) under a positive pressure to prevent oxidation of the tin. The enclosure is designed to allow for the placement of overhead electrical resistance heating elements that maintain the desired temperature gradient of the cooling and stiffening glass ribbon, and water-cooled steel top-of-the-glass rollers assist in the control of thickness and placement of the ribbon of floating glass. The glass ribbon emerges from the float tank through a flame curtain where the H.sub.2 component of the forming gas is burned. As the molten glass spreads on the surface of the metal, nearly perfectly flat and parallel top and bottom surfaces are produced on the glass ribbon. The thickness of the final product is set through control of the mass flow rate of the glass down the bath relative to the rate of glass melt introduction at the hot end of the bath, with attractive force being applied by edge rollers to pull or slow the progress of the glass down the length of the tin bath or by the use of non-wetting (e.g., graphite) barriers that prohibit the melt from spreading to an equilibrium thickness. As noted, the float process has been optimized in the glass industry for a narrow range of NCS glass compositions to produce optical distortion-free architectural and automotive glasses. For the NCS compositions, it has been found that the chemical interaction between the liquid tin and the NCS glass is minimal (i.e., the oxidation and reduction reactions between the two materials are relatively small) at the processing temperatures and conditions that are used in the commercial process.
Because of the high production rates that can be achieved using the float process, it would be desirable to be able to produce float-quality flat glass of other, more complex compositions, for applications such as improved automobile windshields (low infrared transmission) and flat panel display screens (low thermal expansion coefficients). However, the current commercial float process cannot produce such glasses because of inherent mismatches in the basic properties of the tin float bath and the glasses to be floated. For example, glasses with higher melting points than NCS glasses, such as glass-ceramic precursors, require higher float processing temperatures than NCS glasses during both pouring and cooling, as high as 1500.degree. C. At these temperatures, the tin vapor pressure is too high, and tin would be deposited on the top (non-float side) of the glass ribbon. In addition, because the glass-ceramic process requires the homogeneous distribution of transition metal cations (such as Ti.sup.4+ or Fe.sup.2+,3+) and specific oxidation states to achieve the desired crystallization of the precursor glass, the effect of oxidation and reduction reactions and interdiffusion between a very low oxygen chemical potential metal float bath and the oxide glass-ceramic precursor melt would be significant.